A mask pattern of a photomask used for manufacturing a semiconductor device is progressively miniaturized with miniaturization of device patterns. As one of the techniques for increasing the exposure latitude in a severe exposure condition, an SRAF (Sub Resolution Assist Feature) technique is known, which arranges a small pattern near the mask pattern, and the small pattern has a dimension smaller than the resolution limit.
A pattern used in SRAF (SRAF pattern) is smaller than a pattern to be transferred. Therefore, a processing technique for forming the SRAF pattern is important. In addition, a nondestructive pattern cleaning technique is also important to prevent destruction of a fine SRAF pattern during a cleaning step of the photomask.
At present, a strong chemical cleaning process cannot be applied to the photomask since materials of the mask pattern is limited. Therefore, a physical cleaning technique such as a mega-sonic cleaning technique, or a two-fluid cleaning technique is mainly used, where the mega-sonic cleaning technique applies ultrasonic waves to a solution which is obtained by adding additives to pure water based solvent, and the two-fluid cleansing technique cleans the photomask by high-speed and fine liquid particles which are obtained by applying high-pressure air to liquid.
By employing the physical cleaning technique, the cleaning process for the photomask having a thicker SRAF pattern may be performed. This enables preventing a destruction of the SRAF pattern while ensuring cleaning strength for the photomask.
However, the SRAF technique with the thicker SRAF pattern may fail to improve the transfer performance.